You are invited to a special session at the American Geophysical Union (AGU) meeting titled "Mysteries of the Martian Rivers," to be held on Dec. 6 from 2 p.m. to 4 p.m. PST in the Moscone Convention Center (MCC) Theatre, Exhibit Hall C, San Francisco. The session is an oral debate and discussion about the formation of rivers and valleys on Mars. After the session, researchers will be available for follow-up questions from the media in the MCC Press Room between 4 p.m. and 5 p.m. PST. In addition, there will be a poster session on the topic beginning at 8:30 a.m. PST on Dec. 6 in MCC Hall D (P51B).

SCIENTISTS SAY ANCIENT ASTEROIDS, COMETS MAY HAVE CAUSED MARS RAIN

Scientists from NASA and the University of Colorado suggest the bombardment of comets and asteroids on early Mars caused cycles of rain that led to global flooding and the formation of Mars' river valleys and other water-sculpted landscapes.

The researchers emphasize that the period when large comets and asteroids struck Mars appears to correlate with the formation of ancient rivers when water once flowed on Mars, and that both 'events' seem to have ended about the same time, between 3.5 billion and 3.8 billion years ago. The research will be published on Dec. 6 in Science magazine in an article entitled "Environmental Effects of Large Impacts on Mars."

"The river valleys and large craters on Mars may both be about the same age geologically," said Teresa Segura, the paper's lead author. "We think that the two must be related, and our paper describes one possible connection." Segura, a graduate student in atmospheric and oceanic sciences at the University of Colorado, is based at NASA Ames Research Center, in California's Silicon Valley.

The researchers modeled the impacts of asteroids and comets between 60 miles and 150 miles in diameter that bombarded Mars billions of years ago. Such impact events packed a huge energy wallop, equal to about 10,000 million megatons of TNT depending on collision velocities, which were lower back then.

The impacts released water on Mars in four ways, the scientists say - from the vaporized asteroid or comet itself, from Mars' icy polar caps, from the ground where the crater formed and from the heat from hot 'ejecta' (a mixture of soil, rocks and water) that gradually baked water out of the martian soil.

When Mars eventually cooled down after an impact episode, scientists theorize, water that had evaporated into the atmosphere condensed into rain. During Mars' rainy periods, precipitation rates probably averaged between 1 meter and 2 meters a year, similar to Earth's average annual rainfall today.

"This happened dozens of times, maybe more, but after it rained, Mars would go dry," said Dr. Kevin Zahnle, a co-author from NASA Ames. "In the times between impacts, the water sank back into the soil, where it lay dormant until the next time an impact occurred."

Scientists think the martian rains lasted for episodes ranging from months to decades and that, between bombardments, Mars returned to its typical cold, dry state. Besides bringing moisture, the impacts also caused Mars to warm up, they say. During bombardment episodes, hot 'ejecta' from impacts kept Mars' surface warm for hundreds of years at a time.

The martian cratering record shows that there are at least 30 craters carved by impactors that are 100 kilometers or more in diameter. These were created during the planet's period of heavy bombardment by comets and asteroids more than 3.5 billion years ago.

Scientists do not know why a late heavy bombardment stopped about 3.5 billion years ago, according to Segura. "Our research provides some insight into what early Mars might have been like, but we've fit only a couple of pieces into the puzzle of Mars' past," Segura said.

Team members include Dr. Owen B. Toon, University of Colorado, and Dr. Anthony Colaprete, NASA Ames.

The project is funded by the University of Colorado Center for Astrobiology in Boulder, and the NASA Astrobiology Institute (NAI) through NASA Ames Research Center. The NAI is an international research consortium with its central offices located at NASA Ames.

NEW CU-NASA RESEARCH BELIES PREVIOUS IDEA THAT MARS WAS ONCE WARM, WET PLANET

A new study led by University of Colorado at Boulder researchers indicates Mars has been primarily a cold, dry planet following its formation some 4 billion years ago, making the possibility of the evolution of life there challenging at best.

Led by CU-Boulder doctoral candidate Teresa Segura and her adviser, Professor Owen B. Toon, the team used Mars photos and computer models to show that large asteroids or comets hit the planet some 3.5 billion years ago. These impacts apparently occurred about the time major river channels were formed on the Red Planet, said Segura.

According to the available evidence, roughly 25 huge impactors, each about 60 miles to 150 miles in diameter, slammed into Mars roughly every 10 million to 20 million years during the period, blowing a volume of debris equivalent to a global blanket hundreds of yards thick into the atmosphere. The material is believed to have melted portions of subsurface and polar ice, creating steam and scalding water that rained back on Mars at some six feet per year for decades or centuries, causing rivers to form and flow, according to the study.

But the study belies the warm, wet, Mars theory of rivers and oceans embraced by many planetary scientists, since such impactors were so infrequent. "There apparently were some brief warm and wet periods on Mars, but we believe that through most of its history, Mars has been a cold, dry planet," said Segura, currently a visiting researcher at NASA-Ames in California.

A paper by Segura, Toon, CU-Boulder graduate Anthony Colaprete -- now at NASA-Ames -- and Kevin Zahnle of NASA-Ames, will appear in the Dec. 6 issue of Science.

"When the river valleys on Mars were confirmed in the 1970s, many scientists believed there once was an Earth-like period with warmth, rivers and oceans," said Toon, director of CU-Boulder's Program in Oceanic and Atmospheric Sciences and a professor at the University's Laboratory for Astrophysics and Space Physics. "What sparked our interest was that the large craters and river valleys appeared to be about the same age."

In between such catastrophic events, the planet was likely very cold, dry and inhospitable to any life forms, said Toon. "We definitely see river valleys but not tributaries, indicating the rivers were not as mature as those on Earth."

The rare, hot rains pelting Mars that likely came from water in asteroids and comets hitting the planets and the evaporation of some ice from polar caps and ice beneath the impacts would have been spectacular, said Segura. "We believe these events caused short periods of a warm and wet climate, but overall, we think Mars has been cold and dry for the majority of its history."

According to Toon, previous theories that carbon dioxide gas and clouds warmed Mars during its early history "just have not worked out quantitatively." There is no evidence on Mars of large limestone deposits from the first billion years, which would be directly linked to large amounts of C02, a greenhouse gas, he said.

There also is no evidence that another greenhouse gas, methane -- which can be created naturally by volcanic eruptions or produced by primitive life -- was present in the Martian atmosphere. But even CO2 and methane combined would not be enough to warm the planet as greenhouse gases did on Earth and Venus in their early histories, Toon said.

"Hypotheses of a warm, wet Mars, based on the presumption that the valley networks formed in a long-lasting greenhouse climate, imply that Mars may once have been teeming with life," wrote the authors in Science. "In contrast, we envision a cold and dry planet, an almost endless winter broken by episodes of scalding rains followed by flash floods.

"Only during the brief years or decades after the impact events would Mars have been temperate, and only then might it have bloomed with life as we know it," they wrote. Although temperatures in the subsurface of Martian soil may have exceeded the boiling point during the impact period and provided a possible refuge for life underground, the short duration of warm periods predicted by the researchers would have made it difficult for life to ever establish itself on Mars, the team concluded.